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    "## 8.概率图模型\n",
    "+ 每个定点对应联合分布中的一个随机变量\n",
    "\n",
    "+ 有向图（Bayesian network),对于任意一个点$X_i$,设其父节点集合为$pa_i$,那么对于该联合变量来说，给定$pa_i$后，$x_i$与任意非其子节点的点条件独立\n",
    "\n",
    "+ 无向图（Markov network),对任意一条边$(x_i,x_j)$，对于其联合分布，当给定除这两个点之外的节点$(x\\{x_i,x_j\\})$后，$x_i$与$x_j$不条件独立"
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    "## 8.2条件独立\n",
    "+ tail to tail 给定c的条件下,a,b独立\n",
    "+ head to tail 给定c的条件下,a,b独立\n",
    "+ head to head c未被观测时,a,b独立;给定c的条件下,a,b不独立"
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    "### d-划分\n",
    "⼀个结点满⾜下⾯两个性质中的任何⼀个则成为路径被“阻隔”:\n",
    "+ 路径上的箭头以头到尾或者尾到尾的⽅式交汇于这个结点，且这个结点在集合C中\n",
    "+ 箭头以头到头的⽅式交汇于这个结点，且这个结点和它的所有后继都不在集合C中"
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